The invention concerns a central office line interface circuit (hereinafter: COLIC) for a private telephone installation on an analog network. A COLIC connects the switch or key service unit of a private telephone installation to a local central office, which is usually part of a telephone network, to enable telephones or terminals connected to the private installation to communicate with telephones and terminals connected to the network but not connected to the installation.
At present many network subscriber lines are still of the analog type and transmit speech signals in analog form on two wires in an audible frequency band between 300 and 3 400 Hz. These lines can also be used to transmit digital data which is encoded so that it can be transmitted in the frequency band defined above, for example by two-frequency or multifrequency encoding. The two line wires are also used to transmit signalling, for example by modification of the impedance or in the form of a voltage or current or pulses at specified frequencies.
Existing private telephone switches usually employ time switching with the result that the speech signals are coded in digital form. This type of switching is well suited to the transmission of speech and data signals on the same media according to the changing requirements of users.